By default, R Markdown employs inch as a unit of figure size. You can change it by hook_figure_unit.
CAUTIONS
fig.width and fig.height in inches.NULL to the fig.retina chunk option.hook_figure_unit() changes to the unit of figure size to millimeters. Note that this hook will not change the default size.
You may change default unit by the first argument, unit.
Choices are followings: